Advancements in semiconductor manufacture have led to increases in the density and miniaturization of microelectronic circuits. As an example, the manufacture of 64 Mb DRAMs is now possible and 256 Mb prototypes are currently being developed. A key requirement for achieving such high device packing density is the formation of increasingly smaller components. One way to make such smaller components is to employ thinner and smoother films when fabricating those components.
Currently in the art, silicon dioxide films are pretreated with hydrogen ions to prepare the surface of the silicon dioxide film for the deposition of a layer of polycrystalline silicon to provide for a thinner and smoother polycrystalline silicon film. The silicon dioxide is pretreated by ion beam bombardment by a Kaufman ion source. Hydrogen ion beam pretreatment is typically performed using a Kaufman ion beam source directed normally to the substrate. A Kaufman ion source employs a metal grid to accelerate ions at a particular target. During an ion implantation process using a Kaufman ion source, metal from the metal grid sputters off of the grid and becomes implanted in the target object causing the target object to become contaminated. As the size of devices on the target object decreases, the effect of damage caused by sputtered metal from the metal grid increases.
Plasma source ion implantation (PSII) has been used to dope various materials, such as tools, aluminum cans and artificial joints, to improve their wear, friction and corrosion properties. PSII is a process by which ions are implanted into a target at energies high enough to bury the ions below the target's surface. To implant the ions in the target, an ionized plasma is formed about the target in an enclosed chamber. A high voltage pulse is applied to the target relative to the conductive walls of the chamber. Ions from the plasma are then driven into the surfaces of the target from all sides simultaneously without any manipulation of the target.
U.S. Pat. No. 4,764,394 to Conrad teaches one method and apparatus for PSII. Conrad describes that plasma source ion implantations may be performed on complex three-dimensional objects formed from materials such as pure metals, alloys, semi-conductors, ceramics and organic polymers. Conrad describes the process as providing significant increases in surface hardness of metals and ceramics and providing changes in the optical properties and electrical conductivity of organic polymers.
U.S. Pat. No. 5,354,381 to Sheng teaches a plasma immersion ion implantation apparatus which generally is a variation of the apparatus taught by Conrad. The Sheng apparatus uses a pair of power supplies and very short ionization negative pulses applied to the cathode underlying the target in conjunction with or followed by short ionization pulses applied to a second cathode which is facing toward the primary (target) electrode to provide neutralizing electrons.
Thus, a need has developed in the art for a process by which silicon dioxide films can be pretreated to ensure that a subsequently deposited polycrystalline silicon film will be provided with a smooth morphology but without the contamination problems of present processes.